Resonators are devices which have transfer characteristics allowing a very narrow frequency bandwidth signal to pass unattenuated. Typically, such devices are used extensively in tuning circuits for television and radio receivers, as filters in frequency/spectrum analyzers and in high precision oscillators.
In a very narrow bandwidth filter or resonator, such as those used in spectrum analyzers, it is necessary to determine the exact center frequency of the filter or resonator.
The method currently used to measure the center frequency of such as resonator is to drive the resonator with a signal of a constant amplitude and increasing frequency, also known as a sweeping signal. The output signal of the resonator is simultaneously measured. As the frequency of the sweeping signal approaches, passes throiugh, and goes beyond the center frequency of the resonator, the output signal grows, peaks and diminishes. The frequency at which the peak output is reached (i.e. point of minimum insertion loss) is labeled the center frequency.
This method is inadequate for an increasing number of commercial and scientific applications. Primarily, the method is inaccurate and time consuming. It is inaccurate in that the measurement is subjective. The peak signal level at the resonator output may appear to be any frequency of a band of frequencies in and around the actual resonant frequency. To increase the accuracy of that method, the measurement is repeated several times to obtain an average value. It is time consuming in that the frequency sweeping cannot be performed rapidly. If the sweep is too fast, the resonator has a tendency to ring thereby distorting the measurement. This coupled with the repetitions for accuracy further compounds measurement time delays.
It would be desirable to have a method and apparatus for rapidly measuring the resonant frequency of a narrow band resonator with a high degree of accuracy. The method and apparatus of the present invention achieves those desired results.